The present invention relates to a capacity equalizing apparatus for secondary batteries.
In recent years, attention is being given to an enclosed nickel-metal hydride battery (hereafter referred to as a “nickel-hydrogen battery” in this description) being excellent in basic characteristics, such as energy density, output density, cycle life. Development has been advancing to make this nickel-hydrogen battery practical as a power source for motors and as a drive source for various loads in electric vehicles, such as pure electric vehicles (PEVs) and hybrid electric vehicles (HEVs). In the case that this nickel-hydrogen battery is used as a power source for electric vehicles, a battery capacity of approximately 50 to 120 Ah and a total voltage of approximately 100 to 350 V are required to obtain a predetermined drive output. The output voltage of a cell (one nickel-hydrogen cell), the minimum unit constituting the nickel-hydrogen battery, is approximately 1.2 V. Hence, an assembled battery comprising multiple battery blocks, each battery block having one or multiple cells and the battery blocks being connected in series, is used to obtain a desired total voltage.
The temperatures of the cells constituting the assembled battery are not uniform. In particular, in an environment in which the assembled battery is used in a vehicle, temperature differences are apt to occur among the cells. Furthermore, the charging level and the charging efficiency (the ratio of the charged electric quantity to the supplied electric quantity) are different with each cell depending on production process and usage conditions after the production. For these reasons, the cells constituting the assembled battery have variations for each cell in the actual charged state (the ratio obtained by subtracting the discharged electric quantity from the fully charged state of the cell) and the actual capacity (dischargeable electric quantity). If the assembled battery being varying in cell capacity as described above is charged and discharged repeatedly as one unit, cells having small capacities are overcharged or over discharged, and the variation in the capacity among the cells increases. As a result, the range of the capacity being usable as the capacity of the assembled battery is getting narrow. In other words, the service life of the assembled battery is apparently shortened significantly. Hence, for the purpose of preventing the cells constituting the assembled battery from being overcharged or over discharged, it is important to carry out a control to equalize the capacities of the cells or the battery blocks of the assembled battery.
Japanese Laid-open Patent Application Hei 6-253463 has disclosed a battery charging method in accordance with a conventional example, wherein multiple batteries are connected in series and charged. In this method in accordance with the conventional example, a battery pack (assembled battery) comprising batteries connected in series is first fully charged. Then, the voltage of each battery is detected, and a battery having a high voltage is discharged so that the voltage differences among the batteries become small. This prevents the voltage of either one of the batteries from becoming abnormally high and prevents the battery from being overcharged. Since the voltage of a battery is nearly proportional to its capacity, the variation in the capacity among the batteries is equalized.
Furthermore, Japanese Laid-open Patent Application Hei 6-253463 has also disclosed a circuit in accordance with a conventional example, wherein a discharge means comprising a discharge resistance and a discharge switch connected in series is connected in parallel with each battery to individually discharge each of the batteries constituting the battery pack. A microcomputer detects the voltage of each battery on the basis of a signal input to a voltage detection terminal connected to the positive electrode terminal of each battery. The microcomputer then directly on-off controls the discharge switch.
In an electric vehicle, a high-voltage circuit including an assembled battery is insulated from the chassis to prevent risks. On the other hand, a low-voltage circuit including devices, such as a microcomputer for controlling the charging and discharging of the assembled battery, uses the potential of the chassis as a reference potential. Hence, it is necessary to provide a circuit configuration wherein the high voltage generated by the battery blocks or cells constituting the assembled battery is not applied directly to the low-voltage circuit including devices, such as the microcomputer.
In the case that the circuit in accordance with the above-mentioned conventional example is installed in an electric vehicle, for the purpose of providing sufficient insulation between the low-voltage circuit and the high-voltage circuit and transmitting signals from the low-voltage circuit to the high-voltage circuit, it is necessary to use, for example, large and expensive transmission devices, the input and output terminals of which are insulated from each other. The transmission device is, for example, a photocoupler, the light-emitting diode and the light-sensitive diode of which are insulated from each other. In the above-mentioned conventional example, since the number of the transmission devices, the input and output terminals of which are insulated from each other, is required to be equal to at least the number of the battery blocks or cells, a capacity equalizing apparatus including the circuit becomes high in cost and large in size.
For the purpose of solving the problem encountered in the above-mentioned conventional example, the present invention is intended to provide a small and low-cost capacity equalizing apparatus for equalizing the capacities of battery blocks constituting an assembled battery, each battery block comprising one or multiple secondary batteries and the battery blocks being connected in series.